Quality characteristics of water and milk kefir produced with loquat fruit at different extent of maturity

Loquat fruit (Eriobotrya japonica L.) is a subtropical fruit belonging to the Rosaceae family. It is cultivated in Antalya and Mersin provinces in the Mediterranean region in Turkey, and the Ayva Göbek variety grown in Alanya district of Antalya has received a geographical indication as Alanya Loquat. Loquat is a fruit with high nutritional value and rich aroma. However, its shelf life after harvest is short; therefore, it is important to preserve its nutritional value and transform it into high quality and long shelf-life products to increase the consumption of the fruit. In this thesis, 8 different water kefir samples were produced using green (Y), yellow (S) and orange (T) colored loquat fruits with different ripeness and lyophilized culture (Method 1) and kefir grain (Method 2). In addition, 4 different milk kefir samples were produced using loquat fruit and lyophilized yeast. The quality characteristics of the water and milk kefir samples were evaluated in terms of pH, brix, color (L*, a*, b*, C*, h°, ΔE*), total phenolic content (TFC), total flavonoids (TF) and antioxidant capacity (AOC). In addition, the changes in the quality characteristics of the samples after storage at 4±1°C for 0, 14 and 28 days were determined. Finally, the sensory characteristics of water and milk kefir samples were determined by consumer taste and preference tests. The pH and brix values of loquat fruit varied according to the ripeness of the fruit, and the highest pH and brix values were observed in ripe orange color loquat fruit (T). During the fermentation process, pH and brix decreased due to microbial activity in water and milk kefir samples, resulting in a low sugar content and acidic fermented end product. During the storage process of water kefir samples at 4±1°C for up to 28 days, pH values decreased but brix did not change with lyophilized yeast (Method 1) and pH increased and brix decreased when kefir grain (Method 2) was used. In milk kefir samples, pH decreased and brix increased during storage, thus increased acidity and sugar content. Production method, fermentation time, fruit variety, and oxidation rate during preparation affect the color quality in water and milk kefir. It was determined that the yellow loquat fruit (S) had the most vivid and yellowish color with the highest C* and b* values during the semi-ripening period. When water kefir YWK and TWK samples (Method 1) and YWK1 and TWK1 samples (Method 2) were compared, it was observed that the samples produced with water kefir grain (Method 2) were closer to the color of the fresh fruit, but after 28 days of storage at 4±1°C, vivid and yellowish color was determined in the samples with both methods. The darkest, vivid and yellowish color was obtained from milk kefir samples in TMK. L* decreased and a* increased for milk kefir samples during the storage period, resulting in darker and reddish color. TFC, TF and AOC values of loquat fruit harvested at different ripening stages were determined in the range of 59,85-70,71 mg GAE/100 mL, 4,65-9,25 mg RE/100 mL and 26,85-44,92 mg TE/100 mL, respectively, and the highest TFC, TF and AOC values were determined in the Y sample. In addition to the method applied, the degree of ripeness of the fruit, whether the fruit is with or without peel, and the sugar content affect the TFC, TF and AOC ratio in water and milk kefir. It was observed that the water kefir sample produced by the second method (TWK1) had the highest TFC, TF and AOC values. However, there was no difference in the AOC of SWK and KTWK samples with TWK1. In contrast to water kefir samples, the highest TFC and TF in milk kefir samples were found in the unripe green color YMK sample. In general, there was a significant decrease in TFC at the end of 28 days shelf life in the samples produced by the second method compared to the first method in water kefir samples, but an increase in TFC was observed in milk kefir samples, which is related to the fact that milk is an important source of phenolic substances. A decreasing trend in AOC was detected in water kefir samples at the end of 28 days storage, and a similar result was observed in milk kefir samples. When the sensory characteristics of kefir samples were evaluated, the most preferred water and milk kefir samples were SWK with 18.4% and YMK with 33.75%, respectively. The samples with high acidity, sweetness and dark yellowish color were liked and it was determined that the results of the consumer preference test were compatible with the overall acceptance evaluation. With this study, the evaluation potential of loquat fruit in water and milk kefir production was determined.